Side extensions offer a practical solution for adding extra living space, particularly when rear extensions aren't feasible due to limited space or there’s underutilised land along the side of your property.

However, since side extensions are often more visible from the street, they are subject to stricter permitted development and planning regulations compared to rear extensions.

If you're considering a side extension and want clarity on your planning options, this guide walks you through when permitted development applies, when full planning permission is required, and the key rules and considerations for both. It also offers advice on other important factors you’ll need to consider in parallel, such as rights of light, ensuring you're well-prepared to navigate the planning-stage design process.

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Many single-storey side extensions can be built under permitted development. Rear-side extension project by Tuckey Design Studio. Photo by James Brittain.

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Side Extension Permitted Development Rules

Permitted development (PD) rights for side extensions only apply to ‘dwellinghouses’ which are essentially single resident houses. For this specific case, they do not apply to:

• Flats, maisonettes or HMOs.
• Houses on article 2(3) land (conservation area, AONB etc.).
• Listed buildings.
• Houses with covenants or planning approvals removing PD rights.

If your home meets the above criteria, to comply with PD rules, the side extension must not:

1. Exceed single storey, two-storey side extensions aren’t permitted.
2. Exceed half the width of the original house.
3. Be built on a side elevation that fronts a highway.
4. Extend more than 3 metres from the rear wall of the original house for a semi-detached or terraced house, or 4 metres for a detached house, or 6 metres for a semi-detached or terraced house, or 8 metres for a detached house with prior approval (England only).
5. Extend beyond the front elevation of the original house.
6. Adjoin a rear extension (i.e. forming a wraparound extension).
7. Exceed 4 metres in height or the ridge of the original house.
8. Exceed 3 metres at the eaves if it is within 2 metres of a boundary or the eaves of the original house.
9. Exceed 50% of the area of the original site (existing extensions and garden buildings must be included in the calculation).
10. Be built from external materials different in appearance to the original house.
11. Include verandas, balconies or raised platforms.

Refer to the image below for numbered references matching the list above.

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Side extension permitted development rules (front view of house).

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Note that when a new extension adjoins an existing one, PD rules apply to the entire structure. For example, if you add a new side extension to an existing one that exceeds the permitted height or width, the combined structure will not comply with PD regulations. In this instance, you would need to adjust the existing extension to align with the current rules.

A link to the relevant government legislation can be found here.

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Apply for a Lawful Development Certificate

You can apply for a Certificate of Lawful Proposed Use or Development (CLOPUD) from your Local Planning Authority. This legal document confirms that your proposed development is lawful and doesn't require planning permission.

Whilst optional, a CLOPUD provides peace of mind about your project's legality. It's also particularly valuable when selling your property, as it offers potential buyers proof of lawful development.

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Side Extension Full Planning Rules

While Permitted Development (PD) rights are clearly defined by nationwide legislation, policies for full planning permission are often much less prescriptive and vary from one local authority to the next.

To gain planning approval for a side extension, you need to familiarise yourself with local policy. Common requirements include:

• Respecting existing heights and building lines – including eaves and ridge heights and wall positions.
• Replicating the pitch of the existing roof as far as practical (if pitched roofs form part of your design).
• Minimising the impact on neighbouring properties in terms of overlooking, daylight/sunlight, and noise – the internal layout and associated windows need to be carefully considered.
• Ensuring designs appear subservient to the existing house, so the extension doesn't appear to dominate the host building.
• Avoiding a 'terracing effect' where a side extension visually sits too close to the adjacent house – a common requirement where the street consists of semi-detached or detached properties, necessitating a decent setback.
• Using materials and details that complement the existing building – this can be done either by matching like-for-like or taking a contemporary approach.

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Side extension terracing effect in a semi-detached street: A full-height side extension flush with the front elevation (1) poses more planning risk compared to a smaller, subservient extension set back from the front (2).

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Carry out a planning search of similar projects in your local area and analyse the officer's report (all available via your council's planning website) to understand why a particular application was granted or refused approval.

Applications approved under the latest policies carry the most weight. For example, if your local plan was adopted in September 2020, look for applications approved after this date. Older applications are still a 'material consideration', especially if current householder guidance is similar to previous iterations; they just carry a little less weight.

Engaging a trained design professional, such as an experienced residential architect, will help you navigate the planning system more effectively and improve your application's chances of success.

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Side extensions, even if single-storey, require planning permission of they face the public highway. Project by Benjamin Hale Architects. Photo by Pierce Scourfield.

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Rights of Light – A Critical Early Design Consideration

When planning your side extension, you must think carefully about how it might affect your neighbour's windows and their Right to Light.

This legal right is activated after a window has enjoyed uninterrupted light for 20 years or more. It means your neighbours can challenge your plans if they think your extension will significantly reduce the daylight to their property.

It's important to know that even if you get planning permission or can build your extension under PD, you're not automatically protected from legal challenges. Your neighbours can still go to court to try to make you reduce the size of your extension or pay them compensation if your build seriously affects the light coming into their property.

To prevent potential issues, address these considerations early in the design process. Commissioning a daylight report can help evaluate how your extension might impact your neighbours' natural light. By incorporating thoughtful design and maintaining open communication with your neighbours, you can avoid costly disputes and nurture positive relations.

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All two-storey side extensions require planning permission and carry a higher risk of infringing on rights to light unless positioned a good distance from neighbouring properties. Project by Matheson Whiteley. Photo by Maris Mezulis.

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Other Considerations

While planning rules and right of light are the main focus for side extensions, you may also need to consider the Party Wall Act if building close to a neighbouring property, as well as building regulations approval.

Party Wall Act

The Party Wall etc. Act 1996 governs work affecting shared walls, boundaries, or structures between neighbouring properties in England and Wales.

Neighbours can't use it to stop your project, but can cause delays. Early engagement is therefore recommended.

Building Regulations

The Building Regulations set out safety and construction standards, covering aspects such as structural stability, fire safety, heating, electrics, energy efficiency, and drainage.

Building Regulations approval is always required when constructing an extension, regardless of the planning status. Developing a strategy to meet these technical requirements early on will give you confidence that the planning scheme is feasible within the constraints of the site.

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The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.

About the Author

Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.

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